1. Field of the Invention
The present invention relates to a determining method for lattice points for correspondence definition data creation, an image processor, an image processing method, and a medium with an image processing program recorded thereon.
2. Description of the Prior Art
Image devices, such as displays and printers, usually use color image data which renders the color of each pixel using specific color components with gradations. Colors are defined in various color spaces to obtain image data. Such color spaces include, for example, RGB color space and CMY-based color space. In the RGB color space, three colors, R (Red), G (Green), and B (Blue) are used. In the CMY-based color space, C (cyan)-based colors, M (magenta)-based colors, and Y (yellow)-based colors (including lc (light cyan, lm (light magenta), DY (dark yellow), and K (black)) are used. In general, these colors are equipment-independent colors which are specific to each image device. Therefore, to output the same images in the same colors with different image devices, LUTs (look-up tables) are used. LUTs define color correspondence between devices.
It is unrealistic to define correspondence in LUT with respect to all the colors each image device can output because of storage capacity, workability in generating LUT, and the like. Usually, correspondence is defined with respect to a specific number of representative colors, and correspondence is computed by interpolation with respect to any other color. That is, an immense number of colors are not subjected to color measuring. Instead, colors are outputted from image devices to the extent that color measuring is actually performable. Then, color measuring is performed to define LUTs with respect to a specific number of representative colors.
To generate LUT, a specific number of these colors to be subjected to color measuring must be determined in advance. That is, a plurality of lattice points in color spaces must be determined. One of conventional lattice point determining methods is color separation. In color separation, cubic lattice points are defined in the RGB space, for example. Then, the RGB value of each lattice is subtracted form “255”, the maximum gradation value of each color, to obtain virtual CMY values. The number of 3 of CMY colors is not matched with the number of inks in printing devices. Therefore, lattice points whose components are ink colors are determined by defining a specific correspondence for conversion and converting the three colors into six colors or like means. Lattice point determining methods other than color separation include, for example, the art disclosed in Japanese Patent Application No. 2002-2061.
By the above-mentioned lattice point determining method, it is difficult to determine representative colors wherewith color conversion can be performed throughout color spaces with accuracy without local tone jump in gradations. More specific description will be given. When color correspondence is computed using LUT, interpolation or the like is used as mentioned above. Therefore, with respect to colors other than the specific number of representative colors, color conversion accuracy, balance between colors (especially, gradations), and the like greatly depend on how the specific number of representative colors are selected. However, in color separation mentioned above, such processing as conversion of three colors into six colors is performed according to some sort of rule. Therefore, it is difficult to optimize lattice points through color spaces, taking various conditions into account on a case-by-case basis. Especially, with respect to printing devices, various conditions are imposed on specific colors. Such conditions include limitation on ink quantity injectable into paper for printing and limitation on use of K ink for the prevention of grainy appearance. Therefore, it is very difficult to take various conditions locally imposed into account and yet determine the lattice points of representative colors so that tone jump will not occur throughout.